Patent Document 1 discloses an apparatus which alternately irradiates paper sheets being transported with rays of light emitted in different directions from two light sources, and takes two images at the timings of irradiation. In this apparatus, the two images thus obtained are summed into an image, based on which it is determined whether each paper sheet is genuine or not, and of which denomination the paper sheet is. Further, one of the two images is subtracted from the other to obtain an image, based on which it is determined how much the paper sheet is wrinkled.